1. Field of the Invention
The present invention is related to a multi-frequency antenna with a single layer and a single feeding point, and especially to an improved microstrip antenna with its bandwidths simplified and enlarged.
2. Description of the Prior Art
In the initial period of marketing of mobile phones, exposed helix coils are mostly used as the main elements of antennas. Such helix-coil antennas widely used nowadays are generally divided into two main types—contractible and fixed types. No matter which kind of structure is used, an antenna normally has a specific length protruding out of the top surface of the body of a mobile phone. Therefore, various microstrip antennas have been developed, such microstrip antennas are characterized by planeness, concealment and non occupying too much volume.
Among modern planar inverted F-antennas (PIFA), dual-frequency antennas (IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL 45, NO.10, OCTOBER 1997) are of an ideal type of miniaturized microstrip antenna, however, by the fact that:             Electrical      ⁢                           ⁢      volume      ⁢                           ⁢      of      ⁢                           ⁢      an      ⁢                           ⁢      antenna              frequency      ⁢                           ⁢      band      ×      gain      ×      efficiency        =      a    ⁢                               ⁢                             ⁢    constant  
So long as the antenna is made planar and miniaturized, its bandwidths and efficiency of radiation will be reduced and will be necessary to be improved.